With the advent of digital imaging, a number of different writing technologies have been employed for imaging onto photosensitive media such as photographic paper and motion picture film. Early printers adapted cathode ray tubes (CRTs) for providing exposure energy onto the photosensitive medium. In a CRT-based printer, the digital image data is used to modulate the CRT, providing exposure energy by scanning an electron beam of variable intensity along a phosphorescent screen. While CRT-based imaging provided a suitable solution for some imaging applications, high cost and relatively slow speeds, as well as constraints on resolution and contrast limit the usefulness of this approach.
Alternative writers employ lasers, such as the laser-based writing engine disclosed in U.S. Pat. No. 4,728,965. With this type of imaging, red, green, and blue lasers provide the exposure energy. Digital data is used to modulate laser intensity as the beam is scanned by a rotating polygon onto the imaging plane. Unfortunately, as with CRT printers, laser based systems tend to be expensive, particularly since the cost of blue and green lasers remains high. Additionally, compact lasers having sufficiently low noise levels and the stable output behavior necessary for accurate reproduction of an image without unwanted artifacts are not widely available. While lasers provide advantages for high-power applications, they are not well suited to the reciprocity characteristics of conventional photographic film and paper media. Thus, special media is often required for imaging using laser exposure energy.
The speed, cost, and performance problems of existing digital imaging systems limit the utility of such systems with some types of photosensitive media. These problems are particularly pronounced for high speed raster printing applications, such as those needed when printing motion picture film. CRT printers, for example, require exposure durations lasting as long as a few minutes per frame. Commercially available raster laser print systems are faster, but still require 3-10 seconds per frame. For digital mastering of a full length feature film to be commercially feasible, however, print speeds of at least two frames per second are required. Writing film at real-time speeds would require significantly better throughput, at approximately 24-30 frames per second. As the figures given here indicate, this 24-30 frames per second speed appears to be out of reach of conventional CRT and laser technologies using raster imaging methods.
Two-dimensional spatial light modulators, while originally developed for projectors and displays, also show promise for writing applications. Unlike the slower, raster-scanned energy sources of CRT and laser-polygon devices, spatial light modulators provide exposure energy for a complete image frame at a time. Essentially a two-dimensional array of light-valve elements, with each element corresponding to a pixel, the spatial light modulator operates by selectively reflecting or changing the polarization state of each image pixel. Standard types of spatial light modulators include digital micromirror devices (DMDs) and liquid crystal devices (LCDs).
DMD solutions, such as that shown in U.S. Pat. No. 5,461,411 offer advantages such as longer exposure times compared to laser/polygon writers. This helps to alleviate reciprocity problems associated with photosensitive media during short periods of light exposure. However, DMD technology is expensive and is not widely available. Furthermore, DMDs are not currently available at high enough resolutions for printer applications and are not easily scaleable to higher resolution.
The LCD type of spatial light modulator modulates by selectively altering the polarization state of each image pixel. Types of LCD include transmissive and reflective. Both types of device have been used in imaging systems. For example, U.S. Pat. Nos. 5,652,661 and 5,701,185 disclose printing apparatus that form images using transmissive LCDs. However, there are several drawbacks to the use of conventional transmissive LCD technology. Because of the space required for circuit traces and components, transmissive LCD modulators generally have reduced aperture ratios. Transmissive field-effect-transistors (TFT)-on-glass technology does not provide sufficient pixel to pixel uniformity needed for many printing and film recording applications. Furthermore, the large footprints of transmissive LCD devices, needed in order to provide large numbers of pixels for improved resolution, prove to be unwieldy as part of an optical system designed for printing or film recording applications. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes.
In contrast, reflective LCD modulators provide superior performance and significantly reduce the cost of the printing system. Exposure times for individual pixels shift from tens of nanoseconds to tens of milliseconds, a million-fold increase. This increased exposure time, along with an increased aperture size for each pixel, allows a modest increase in writer throughput to two frames per second or better, without the media reciprocity problems otherwise caused by short exposure times.
Both transmissive and reflective types of LCDs present limitations. However, particularly with reflective LCDs, device performance continues to improve, making it advantageous to employ reflective LCDs in printer applications.
LCDs can modulate light from any of a number of sources. Conventional printers employ lamps as light sources. However, although lamps can provide output power levels sufficient for high speed printing, they do have some inherent limitations, such as high levels of heat and IR generation. Additionally, lamps cannot be switched on and off at rates sufficient for high speed print applications. For reasons cited above, lasers continue to be less desirable for writing to photosensitive media. In addition, laser sources exhibit undesirable coherent light effects, such as speckle. Light emitting diodes (LEDs) have been used, with some degree of success, in printers, however, there is considerable room for improvement.
It is well recognized in the imaging arts that LEDs are not designed with writing applications in mind. Conventionally manufactured for use in display applications with visible light, LEDs are designed to provide modest levels of brightness with high angular divergence. Since they are characterized primarily for display functions, LEDs are specified by manufacturers for brightness and other photometric characteristics that relate to human eye response. Photometric ratings of LED light sources, usually given in lumens or candelas, give very little useful information related to suitability for film exposure.
It is the radiometric, rather than photometric, characteristics of the light source that are of most interest when writing onto photosensitive media. Writing wavelengths, for example, need not be in the visible range; there can be advantages in using light with very short, near-ultraviolet wavelengths or in longer infrared wavelengths. Writing speed, for example, is a function of radiance (typically expressed in watts per square centimeter-steradian), rather than of brightness. The high angular divergence of light from LEDs, beneficial for display purposes, is detrimental for writing purposes, where a narrow emission angle and small emission area works best for achieving high resolution and high speed. Specifications of dominant wavelength, conventionally provided for high-brightness display LEDs, can be misleading from the perspective of characteristics needed for writing to photosensitive media. The actual peak emission wavelength can vary substantially from the dominant wavelength that is perceived by the human eye.
Photosensitive Media Characteristics
As is described above, LEDs are largely designed for visibility to the human eye, for use in a variety of display applications. Design for display visibility, however, often conflicts with requirements for exposure of photosensitive media. Referring to FIG. 3, there is shown a representative graph of spectral sensitivity versus wavelength per color layer for a type of color internegative film used in motion picture printing. The vertical scale is a log scale. Thus, a 0 value corresponds to an exposure level of 1 erg/cm2; a value of 1 corresponds to an exposure level of 0.1 erg/cm2; a value of −1 corresponds to an exposure level of 10 erg/cm2. For comparison, overlaid onto graphs for spectral sensitivity of this photosensitive medium is a normalized curve showing the photopic response of the human eye. As the graph of FIG. 3 indicates, there can be substantial differences between the response of a photosensitive medium over a range of wavelengths and the response of the human eye. Certainly, the human eye has a heightened response to subtle changes of color intensity and hue in some parts of the spectrum. Film sensitivity, however, is much more pronounced for different exposure wavelengths. As shown in FIG. 3, for example, sensitivity for the red layer is roughly 1/100 that of the blue layer.
The color response characteristics of photosensitive media often differ from response of the human eye within the same region of the spectrum. For example, the ideal blue wavelength for film exposure is approximately 450 nm, a value near the peak of the blue curve and near the minimum value of the green curve. Note that, for visibility to the human eye, however, a value of 480-490 nm would be much preferred. With the film sensitivity shown in FIG. 3, however, a 490 nm exposure would not be appropriate, since it would affect both blue and green layers to somewhat the same degree. Similarly, for the red layer, an optimal wavelength for the film would be in the 685-695 nm region. However, this wavelength may be difficult to perceive, being very near the edge of the visible spectrum. Note that a red LED, with a dominant wavelength typically near 625 nm, is optimal for traffic light or other visible uses. However, for film exposure, almost 4 times the amount of light is needed at this wavelength than would be required at 690 nm. Thus, it can be seen that the sensitivity of photosensitive media to wavelengths may even conflict with human eye response, for which LEDs are primarily designed.
Clearly, there are significant differences between what is required of the LED as a light source for display versus what is required as an exposure source for imaging onto photosensitive media. Because LEDs have been primarily developed for display applications, special techniques are required to adapt these devices to a high-speed writing apparatus.
LED Composition and Characteristics
FIGS. 1a, 1b, and 1c show, from top, side, and cross-sectional perspective views respectively, the construction of a conventional encapsulated discrete LED 32 used in indicator lamp and other display applications. Referring to FIG. 1a, a bond wire 27 is connected to an electrode 30 on an LED chip 25. For a typical device, discrete LED 32 is approximately 200 to 250 um square, with electrode 30 having a diameter in the 120 to 150 um range. Necessarily, electrode 30 covers a substantial portion of the light emitting area of discrete LED 32 in the desired emission direction. Referring to FIG. 1b, in which LED chip 25 is soldered to a substrate 29, the preferred emission direction for LED light is shown. However, as indicated in FIG. 1b, a considerable amount of light is emitted in undesirable directions, from the edges of LED chip 25. This is due, for example, to the reflective effects of the solder and to some reflectivity of electrode 30 itself. Bond wire 27 also blocks some amount of light. It is difficult to collect and use light emitted from these edges. In fact, because of the position of electrode 30, only a small amount of light is actually emitted along the preferred axis. The optical axis itself can be darker than off-axis areas.
Referring to the cross-section of discrete LED 32 shown in FIG. 1c, LED chip 25 is positioned within a reflector cup 24 which helps to collect some of the light emitted in an undesirable side direction and direct this light vertically in the desired direction. The conventional discrete LED 32 is encapsulated in an epoxy dome lens 28, which helps somewhat to direct light in the desired direction. Drive current is supplied from an anode lead 21, through a thin gold bond wire 27, then through LED chip 25, to a cathode lead 23. For cooling, convection and heat radiation are negligible. Instead, heat generated within discrete LED 32 must be conducted from discrete LED 32 by cathode lead 23. High thermal resistance reduces overall LED power and device lifetime.
The conventional design approach used for discrete LED 32, as shown in FIGS. 1a, 1b, and 1c, is acceptable for many display applications. However, as is noted in the above description, due to sensitometric response characteristics of photosensitive media and to drawbacks of conventional discrete LED 32 design, the conventional approach is relatively inefficient and is not well-suited to writing applications. Some improvements can be made in the design of discrete LED 32 devices. For example, referring to FIGS. 2a and 2b, there are shown top and side views, respectively, of an improved design for discrete LED 32. Here, two electrodes 30 are positioned in diagonal corners of LED chip 25, thereby eliminating the on-axis dark spot of convention discrete LEDs 32 and allowing increased light emission in the desired direction of the optical axis. In addition, drive current is more uniformly distributed within LED chip 25 with the arrangement of FIGS. 2a and 2b. However, thermal build-up remains a problem that is not sufficiently alleviated with the design solution of FIGS. 2a and 2b. 
Solutions for Grouping LEDs
One method for obtaining higher levels of exposure energy from discrete LEDs 32 is to group together multiple discrete LEDs 32. The relatively small size of discrete LED 32 components makes this approach feasible within some limits. Referring to FIG. 4a, there is shown one example of a 3×3 array of discrete LEDs 32, in an arrangement of colors. Such an array of discrete LEDs 32, assembled on an LED mount plate 31, can be deployed to provide increased brightness where it is advantageous to use discrete LED 32 sources. As FIG. 4a shows, it may be advantageous to arrange discrete LEDs 32 in various patterns and to have relatively more discrete LEDs 32 of some colors, based on the spectral sensitivity of the photosensitive medium being exposed.
In a typical writing apparatus, the acceptance cone angle of the illumination system determines within what area discrete LEDs 32 can be placed on LED mount plate 31. In practice, suitable illumination optics typically have an acceptance cone angle of about f/4. With conventional lens components and design approaches, such as using a 25 mm diameter achromatic lens with a 100 mm focal length, for example, this would constrain the available area, requiring LEDs to be positioned within approximately a 1 inch square.
Because of simple light cone geometry, there are some limitations to light efficiency in providing multiple discrete LEDs 32 within a narrow illumination aperture, as shown with respect to FIGS. 5a and 5b. In FIG. 5b, discrete LED 32 emits light having an overall LED divergence angle 33 that exceeds an optics acceptance cone angle 34 of a collimating lens 36 for the illumination optics. A waste light portion 35 of emitted light from discrete LED 32 lies outside optics acceptance cone angle 34 and is, therefore, unusable within the printing apparatus. Where multiple discrete LEDs 32 are deployed, even where the emission cone angle is smaller than the optics acceptance cone angle, there can still be considerable unused light, as shown in FIG. 5a. Here, discrete LEDs 32 on the outskirts of LED mount plate 31 have an increased amount of unused light, making a larger contribution to waste portion 35 than do more centrally located discrete LEDs 32. Thus, simply increasing discrete LED 32 density has its limitations for providing increased exposure energy. Solutions using smaller discrete LED 32 components may provide a modest increase in exposure energy, but are also subject to similar inefficiencies due to angular divergence.
An alternate approach for providing sufficient exposure energy is the use of large area LED devices having patterned electrodes, as shown in FIGS. 7a and 7b. This approach provides, from a relatively small illumination area, the equivalent energy of an array of discrete LEDs 32. Commercial versions of compact large area LED devices include Luxeon Star devices available from Lumileds Lighting, LLC, located in San Jose, Calif. or similar components available from Cree, Inc., located in Durham, N.C., for example. Referring to FIG. 7a, one type of large area LED device, a patterned electrode LED 134, has a patterned electrode 40 that spreads current uniformly through LED chip 25 without substantially obstructing emission in the desired direction. FIG. 7b shows a side view of patterned electrode LED 134. By way of comparison, LED chip 25 used in patterned electrode LED 134 is approximately 2 mm on an edge, about ten times the length of a standard LED chip 25 as used in a conventional discrete LED 32. However, patterned electrode LED 134 is still small enough to be sufficiently collimated by a single lens. A custom lenslet array is not required.
FIG. 8a shows, in more detail, how patterned electrode LED 134 such as the Luxeon Star device is constructed. A large area LED 46 is mounted on a metal heatsink 48 and is connected externally by electrical leads 43 and internally by bond wires 27. The device is covered by a clear plastic lens 128. As shown in FIG. 8b, auxiliary collector optics can be added to increase light intensity from patterned electrode LED 134. A collector cone 41, such as a plastic molded component, serves as a light guide and provides both collimating and reflective optics for directing the emitted light. Collector cone 41 comprises an integrally molded collimating lens 36 to aid in collimating emitted light received at its input end to provide narrower angle emission. Additionally, collector cone 41, as a prism structure, acts as a guide element using total internal reflection (TIR) to help redirect the wider angle emissions. It can be seen that the patterned electrode large area LED goes a long way towards providing a light source for film imaging applications, providing relatively high radiance, high power, small emission angle, and small source area, and having reduced tendency for thermal buildup and absence of a “dead spot” on axis.
While solutions such as clustering LEDs and combining patterned electrode LED 134 with collector cone 41 provide some improvement for achieving high levels of exposure energy from LED sources, it can be appreciated that significant difficulties remain. In order to provide sufficient exposure energy for printing at efficient speeds, even more intense illumination energy is needed. At the same time, however, this energy must be emitted from a small source, from within a limited area, and at low divergence angles.
Conventional approaches currently allow writing speeds of up to about 1 frame/second. Commercial viability, however, demands speeds approaching 24 frames/second. Because increases in exposure energy translate directly to potential increases in writing speed, even incremental improvements in providing increased exposure energy can be beneficial, provided that the necessary area limitations and divergence angle constraints are met.
Thus, there is a need for an improved printing apparatus capable of high speed printing onto photosensitive media and utilizing high-intensity LED illumination.